Rover 25 ?

my rover 25 (53 plate) is making like a clicking noise when you start it then the faster you drive the faster the clicking gets then it stops after you have been driving for a bit. When I first took it to a mechanic he said he thought it was the beginning of HG failure because the heaters had just broke but then he said it wasnt and done a few things heaters timing belt fan belt etc. but the noise was still there and a few people told me they thought it was the tappits so of it went again and he done the tappits and now after 1 day the noise is back. Can anyone tell me what they think as I dont want to keep getting things done that dont really need to be done (I can't afford to).
Thanks